Very frequently used representatives of endoprostheses are stents (endovascular prosthese) that are used for the treatment of stenoses (vascular constrictions). Stents generally have a basic structure in the shape of a hollow cylinder or tube that is open at both ends. The endoprosthesis is inserted into the blood vessel or body part to be treated, and serves to support it and/or hold it open. Stents usually assume one of two states, namely a compressed state with a small diameter, and an expanded state with a larger diameter. In the compressed state, the stent can be introduced into the blood vessel to be supported, by means of a catheter, and positioned at the location to be treated. For this purpose, the stent is frequently crimped onto a catheter. At the location of treatment, the stent is then dilated, for example by means of a balloon catheter, or makes a transition into the expanded state (when a shape memory metal is used as the stent material) by means of being heated in the blood, above its critical temperature.
The reason for the benefit of implantation of endoprostheses into blood vessels is the greater primary gain in lumen that is produced by the inner volume of the basic structure. It is true that an optimal vascular cross-section that is necessary for therapy success can be achieved by means of the use of such endoprostheses, but the permanent presence of such a foreign body induces a cascade of microbiological processes that can lead to the stent gradually becoming overgrown. For example, tiny injuries, tears, or dissections of the vascular wall are caused during contact of the endoprosthesis with the vascular wall during dilatation, i.e. during widening of the blood vessel, which generally heal without problems, but can lead to excrescences because of the cell growth that is triggered. The permanent presence of an implant also brings about processes that can lead to narrowing in the vascular cross-section, i.e. to restenosis.
For the reasons indicated above, it was previously the goal in the development of stents to achieve the greatest possible radial strength at the lowest possible recoil (elastic rebound). In this connection, radial strength is understood to be the internal resistance of the implant to forces that act radially and can bring about radial compression of the implant, in its expanded state. In this connection, the radial strength can be expressed quantitatively by stating a collapse pressure. In this connection, the compression takes place suddenly when the collapse pressure is reached—the implant collapses.
In the reference EP 1 642 551 A1, an implant is described that is characterized by a lower radial strength than previously known implants. This is because it was determined that a lower radial strength is not only tolerable, particularly for biodegradable implants, for a great number of pathological vascular changes, but also leads to a clear improvement in the healing process. In the reference EP 1 642 551 A1, an implant is therefore proposed in which, proceeding from the expanded state, the cross-sectional area, i.e. the internal volume gradually decreases with an increasing pressure applied radially, until a specific, predetermined pressure value is exceeded. The known implant can therefore be constantly compressed up to an established limit value of the internal volume, i.e. the cross-sectional area. A further increase in the compression pressure does not lead to a further decrease in the internal volume or the cross-sectional area until the compression exceeds the collapse pressure. If the compression pressure is increased further, the implant will collapse. The behavior of such an implant leads to an improvement in the healing progression, but this does not always proceed in satisfactory manner.